This invention relates to a pneumatic power wrench, in particular to a pneumatic power wrench having the specific features stated in the preamble of claim 1.
A general problem concerned with most types of screw joint tightening tools is to actually accomplish the desired pretension level in all joints, irrespectively of difference in torque resistance characteristics of the screw joints. A particular problem of this kind is to avoid undesireable torque overshoot or premature motor shut-off at tightening of stiff or hard joints, depending on whether the wrench is of the stalling type or if it is provided with a retardation responsive shut-off means.
The best way to solve this kind of problem is to reduce the idle or low-load speed of the motor such that the kinetic energy of the rotating parts as well as the retardation magnitude is reduced. A lower kinetic energy adds less tightening torque to the desired target torque level, and lower retardation magnitudes do not cause any premature shut-off in retardation responsive shut-off mechanisms.
One previously known way to solve the above problems is to provide the power wrench with a speed governor which reduces the idle speed level of the motor without impairing the low speed output capacity of the tool. This, however, is a relatively complicated solution to the problem, since it adds a number of details and complicates the power tool design.
Another, simpler and commonly used way to solve this kind of problem is to emplyo a restriction in the pressure air inlet passage to the motor. This results in a reduction of the idle speed of the motor and, accordingly, a reduction in the kinetic energy of the rotating tool parts. However, this solution to the problem also causes a restriction of the low speed power output of the motor, which of course is a disadvantage since the full capacity of the tool is not available.
Still another way of reducing the idle speed of a pneumatic power tool is to restrict the exhaust air outlet flow from the motor. This way is better than restricting the pressure air inlet flow of the motor, because an outlet flow restriction is effective in reducing the idle speed of the motor without impairing the low speed power output of the motor. This is important since it makes it possible to utilize the full capacity of the motor during the final pretensioning phase of a screw tightening process.